The military HF, VHF and UHF bands (2 MHz to 500 MHz) span eight octaves in frequency and provide essential communications for naval vessels and land-based vehicles. These bands are also important for electronic warfare (EW) activities which include Signal Intelligence (SIGINT), Electronic Intelligence (ELINT), Information/Operations (I/O), and Electronic Attack (EA). In order to accomplish these EW functions, signal acquisition and direction finding capabilities are important.
Because of electromagnetic interference (EMI) between co-sited communications transmitters and SIGINT/ELINT receivers, the intelligence gathering functions can be very adversely impacted. To provide all of the desired functions, without being impacted by EMI it is important to provide a radio frequency distribution system for distribution of extremely small SIGINT and ELINT signals while also handling I/O, EA and communications signals. The radio frequency distribution system needs to provide a low noise RF path between the antennas and the processing electronics while operating in a high EMI environment.
A signal intelligence (or electronic intelligence) receiver intercepts radio signals at a high sensitivity across a large bandwidth of the radio spectrum. This ability to intercept radio signals may be compromised by noise sources that effectively reduce the physical range from which the signal intelligence receiver can pick signals of interest from their origins. Frequency-hopping, frequency-scanning wideband and ultra-wideband communications receivers cannot employ simple narrowband pre-selector filters to protect amplifiers and limiters in receiver front ends from strong interference outside the communications signal bandwidth. Close proximity to multiple transmitters reduces the effective communications range of such receivers to almost zero. In particular, frequency hop (FH) transmissions add to the complexity of co-site EMI concerns because they add a time dimension to the spectrum management problem.
This range reduction has been shown to be due, at least in part, to intermodulation products in the front end of the receiver. Diodes near the receiver's antenna port used for power limiting or circuit switching act as mixers. The resulting intermodulation products affect virtually every communications channel in the receiver's range. It should be noted that intermodulation products are produced whenever two or more high-power interference signals appear in the same nonlinear device at the same time.
One method that may be used to reduce cosite EMI effects is antenna-to-antenna isolation. Although antenna isolation may appear to be an easy and effective solution to cosite EMI problems, it is often not a feasible solution because it requires space that is not available.
A second method that may be used to reduce cosite EMI effects is preselector filtering. Using a preselector bandpass or band reject filter can be effective, but with frequency hopping systems it is necessary to use a bank of filters such that the signal hops from filter-to-filter. Also, it is necessary to ensure that nonlinear interactions do not occur after the filters.
A third method that may be used to reduce cosite EMI effects is digital signal processing. Superconducting analog-to digital converters have characteristics which are amenable to cancellation of high-level narrowband signals. Extremely high sampling rates are possible with these devices.
Improvement in wideband reception has been provided through the use of multiple bandpass filters with contiguous passbands. For details, refer to U.S. Pat. No. 6,549,560 to Maiuzzo et al. Although the use of multiple bandpass filters has provided substantial improvements in wideband communication receiver performance, this structure has not been modified for use in the context of a receiver for signal/electronic intelligence. Thus, there remains demand for further increased performance, particularly increased range.
Thus, a continued need exists for front end filtering for wide bandwidth receivers providing reduced bit error rates and/or increased reception range.
In an embodiment, a comb limiter combiner for front end filtering provides for reduced bit error rates with an increased reception range. The comb limiter combiner according to one embodiment has an input signal coupler for coupling to a receiving antenna and distributing the antenna signal to a bank of input bandpass filters that each utilize high temperature superconductors cooled cryogenically. The cryogenically cooled input bandpass filters have contiguous passbands that comprise the total receiver bandwidth. Each input bandpass filter is connected to a limiter having a threshold substantially equal to the limiting threshold of the receiver. Each limiter is connected to a cold low noise amplifier. Each amplifier is connected to an output bandpass filter similar to the corresponding input bandpass filter to remove out-of-band intermodulation products generated by the limiter. Each of the output bandpass filters is cryogenically cooled and uses high temperature semiconductors. The bank of output bandpass filters is connected to an output signal coupler for coupling to the front end of the receiver.
A comb limiter combiner according to another embodiment has an input signal coupler for coupling to a receiving antenna and distributing the antenna signal to a bank of input bandpass filters. The input bandpass filters have contiguous passbands that comprise the total receiver bandwidth. Each input bandpass filter is connected to feed into a linear self-adjusting attenuator, and each attenuator is connected to feed into an automated self-tuning notch filter. Each notch filter is connected to input to a low noise amplifier. Each amplifier is connected to an output bandpass filter similar to the corresponding input bandpass filter to remove out-of-band intermodulation products.
A comb limiter combiner according to another embodiment has an input signal coupler for coupling to a receiving antenna and distributing the antenna signal to a bank of cryogenically cooled input bandpass filters. The input bandpass filters have contiguous passbands that comprise the total receiver bandwidth. Each input bandpass filter is connected to feed into a cold linear self-adjusting attenuator, and each attenuator is connected to feed into an automated self-tuning cold notch filter. Each notch filter is connected to input to a cryogenically cooled output bandpass filter similar to the corresponding input bandpass filter to remove out-of-band intermodulation products.
A comb limiter combiner according to yet another embodiment has an input signal coupler for coupling to a receiving antenna and distributing the antenna signal to a bank of input bandpass filters. The input bandpass filters have contiguous passbands that comprise the total receiver bandwidth. Each input bandpass filter is connected to feed into a low power automated notch filter. Each notch filter is connected to feed into a limiting amplifier. Each of the limiting amplifiers is connected to input to an output bandpass filter similar to the corresponding input bandpass filter to remove out-of-band intermodulation products.
One feature of comb limiter combiners according to these embodiments is the use of cryogenically cooled high temperature superconductor band bass filters.
Another feature comb limiter combiners according to these embodiments is the use of cold notch filters to suppress interference.
An advantage of the comb limiter combiners according to these embodiments is that intermodulation products are restricted to the passband of a single bandpass filter.
Another advantage is that a comb limiter combiner design according to these embodiments requires no knowledge of the frequency excursions of the transmitted signal.
Yet another advantage is that a comb limiter combiner design according to these embodiments requires no switching or control circuitry.